The present invention relates to a method of determining, by means of a clamping-force transducer which is clamped in position instead of the workpiece, the clamping force present in a given condition of clamping in a rotating clamping device for holding a given workpiece fast, taking into consideration the centrifugal force.
Upon the operation of rotating clamping devices, centrifugal force effects occur which act on the clamping elements, which in their turn act radially from the outside on the workpiece in a direction opposite the direction of the clamping force so as to result in a reduction in the clamping force. The amount of centrifugal force depends on the mass of the radially movable clamping elements and the speed of rotation of the clamping device, centrifugal force increasing as the square of the increase in speed of rotation. This increase of the centrifugal force by a power of two correspondingly reduces the clamping force present at standstill upon an increase in the speed of rotation so that the danger exists that, during a chip removal operation, particularly if it is effected by the use of modern cutting tools of high cutting speed, the required holding force of the rotating device for the clamping of the workpiece will no longer be present and the workpiece will be thrown out of the clamping device.
In order to avoid this danger it is known to measure the clamping force, not only with the clamping device stationary but also with the clamping device in rotation, by means of a clamping-force transducer which is clamped in the clamping device instead of a workpiece. As is well known, such transducer may be coupled via slip rings to electrical measurement circuitry for obtaining electric signals during rotation of the chuck and transducer. In this way it has been attempted to determine, before clamping the workpiece to be machined into position, whether the clamping force applied by the clamping device at the operating speed will be sufficient, under the prevailing operating conditions and despite the centrifugal force, to hold the workpiece safely in the clamping device.
In this connection, the fact is disregarded that the holding force of the clamping device upon operation is dependent not only on the rigidity of the clamping device but also on interaction with the rigidity of the specific workpiece.
Starting from the condition of clamping with the clamping device stationary, in which equilibrium prevails between the stresses and strains of the clamping device on the one hand and of the clamped workpiece on the other hand, this condition of equilibrium is changed by the influence of centrifugal force when the clamping device is rotating. Although the absolute value of the centrifugal force depends on the mass of the clamping elements and the specific speed of rotation, only a part of this centrifugal force is used to reduce the clamping which exists at standstill. The remaining part of the centrifugal force is used for deformation, i.e. for the widening of the clamping device.
The ratio between the portion which reduces the clamping force and the portion which does not cause any reduction in clamping force depends, for the same clamping device and speed of rotation, in a specific manner on the rigidity, at the place of clamping, of the workpiece clamped at the time.
The condition of equilibrium which prevails in the clamped condition between the clamping device and the workpiece is obtained only as a result of the fact that the clamping device is able to apply such high forces that the workpiece is deformed within its elastic region, a part of the clamping energy being stored in the form of deformation in the workpiece.
When workpieces of lesser rigidity are in clamped condition with the spindle stationary, relatively high deformation of the workpiece takes place which, upon the widening of the clamping device by the centrifugal force results in subsequent expansion of the workpiece and thus in less loss of clamping force. In the case of workpieces of greater rigidity, on the other hand, despite the application of the same amount of clamping force under the same conditions, less deformation is produced due to the greater rigidity of the workpiece. Accordingly, upon the occurrence of this centrifugal force there is only a small elastic recovery path present so that under the same clamping conditions less residual clamping force remains in the case of rigid workpieces than in the case of soft workpieces. The rigidity of the workpiece, other conditions being the same, is thus of decisive importance for the clamping force which remains under the action of centrifugal force.